Does he actually need toughening up though? He's hardly an 8 stone weakling a la Willo Flood. I'd bet my house that Barker is bigger and stronger than Raheem Sterling, and he was never farmed off to the Championship to 'toughen up'.
He may not be a weakling, but he hasn't played against grown men who will try anything to stop him. For me though, I believe these loans are more about testing the mentality of a player. You see he will have had everything at his disposal at City, but now he is in a strange place with few friends about. The fact he is at a club at the bottom should not matter. What matters will be his application to the task ahead and if he can rise to the challenge in front of him.